PM2.5 are mostly secondary organic aerosols, meaning particles that form after the reaction of other emissions. Unfortunately these can occur even with chemical

PM2.5 are mostly secondary organic aerosols, meaning particles that form after the reaction of other emissions. Unfortunately these can occur even with chemicals that have traveled for days and then they react with local emissions. So if you live in a forest which emitts a load of pinene and other organics and due to microclimate you receive NOX etc from a city not that nearby, boom you get very high PM2.5.